Secondary batteries that are light and have high energy densities have been hitherto progressively developed as power sources of portable electronic devices such as note book type portable computers, portable telephones, video cameras with VTRs (video tape recorders), etc. As secondary batteries having high energy densities, lithium-ion secondary batteries have been developed which use lithium, lithium alloys or materials capable of being doped with/dedoped from lithium ions as anode active materials, and metal oxides or metal sulfides as cathode active materials. In lithium-ion secondary batteries, carbonaceous materials are used for electrochemical, physical and mechanical reasons, and reasons of battery performances, cost, safety, etc.
As for the carbonaceous materials, non-graphitizable carbons having amorphous structures or graphite are used. The graphite includes natural graphite and artificial graphite. The artificial graphite includes spherical graphite, massive graphite, fibrous graphite, etc. When the natural graphite is employed as the anode active material of the lithium-ion secondary battery, the natural graphite can increase a battery capacity. However, the natural graphite is disadvantageously low in its other battery characteristics and hardly treated upon manufacturing a battery. On the other hand, the artificial graphite is easily treated upon manufacturing a battery and large in lithium storage per unit mass or unit volume. Accordingly, the artificial graphite is excellent as the anode active material of the lithium-ion secondary battery.
The spherical graphite among the artificial graphite is called, for instance, a mesophase graphite. Pitch or the like is heated to form a spherulite, what is called a mesophase and an unnecessary part of the mesophase is dissolved by a solvent, heated and graphitized to obtain the spherical graphite. The spherical graphite is also called MCMB, an acronym for mesophase carbon microbeads. The spherical graphite is produced by a method in which, for instance, the spherulite is allowed to grow large and crystallize, then the crystallized product is heated and pulverized, as well as the above-described method.
The above-described artificial graphite exhibits excellent battery characteristics when artificial graphite is used as an anode of the lithium-ion battery. However, the artificial graphite has a problem that a lithium storage per unit mass or unit volume is lower than that of natural graphite.
Further, with spherical graphite, the volume density of the anode is decreased due to spaces generated when two or more spherical bodies are allowed to come into contact with each other, so that a battery capacity is hardly increased.
As a means for improving the volume density of the anode, there is a method in which the range of small particle size in the particle size distribution of the artificial graphite is widened to increase the amount of fine powder. Thus, the fine powder increases the volume density of the anode. Specifically, a method for containing fine particles of 0.3 μm or smaller in an anode active material is proposed in, for instance, Japanese Patent Application Laid-Open No. hei 11-3706. In this case, when the spherical graphite having the fine powder increased is used for the anode, the spherical graphite is high in its reactivity with electrolyte solution and high in its activity because the spherical graphite has a large surface area relative to the volume of the anode. Thus, the battery safety is disadvantageously deteriorated. Although the fine powder is high in its reactivity with the electrolyte solution, a battery capacity is low. Further, when the spaces are excessively filled with the fine powder in the anode, it is feared that there is no space for retaining the electrolyte solution in the anode, and the resistance of the anode side becomes high which deteriorate the battery characteristics. That is, the addition particles that are too small is not preferable for the battery.